Identification of a third variable lymphocyte receptor in the lamprey

Abstract

Jawless vertebrates such as lamprey and hagfish lack T-cell and Bcell receptors; instead, they have unique antigen receptors known as variable lymphocyte receptors (VLRs). VLRs generate diversity by recombining highly diverse leucine-rich repeat modules and are expressed clonally on lymphocyte-like cells (LLCs). Thus far, two types of receptors, VLRA and VLRB, have been identified in lampreys and hagfish. Recent evidence indicates that VLRA and VLRB are expressed on distinct populations of LLCs that resemble T cells and B cells of jawed vertebrates, respectively. Here we identified a third VLR, designated VLRC, in the lamprey. None of the ≈100 VLRC cDNA clones subjected to sequencing had an identical sequence, indicating that VLRC can generate sufficient diversity to function as antigen receptors. Notably, the C-terminal cap of VLRC exhibits only limited diversity and has important structural differences relative to VLRA and VLRB. Single-cell PCR analysis identified LLCs that rearranged VLRC but not VLRA or VLRB, suggesting the presence of a unique population of LLCs that express only VLRC.antigen receptors | immune system evolution | jawless vertebrates | leucine-rich repeats | somatic rearrangement

Definition of a thirdVLRgene in hagfish

“…In comparing the sequences of the two known hagfish VLRs with those of the three lamprey VLRs, we noticed that the highly variable inserts in the LRRCT module of the currently designated hagfish "VLRA" are more similar to those of lamprey VLRC than to those of lamprey VLRA (13,17,18). This led us to hypothesize that a third hagfish VLR, if present, would be the true counterpart of lamprey VLRA.…”

“…Three VLR genes have been identified in lampreys (VLRA, VLRB, and VLRC), but only two (VLRA and VLRB) have been identified so far in hagfish (4,7,8,13). Lamprey VLRB-expressing cells can respond to immunization by undergoing lymphoblastoid transformation, clonal expansion, and secretion of their antigenspecific VLRB antibodies (9,14).…”

Copy number and sequence variation of leucine-rich repeat modules suggests distinct functional constraints operating on variable lymphocyte receptors expressed by agnathan T cell-like and B cell-like lymphocytes

“…VLRs generate diversity comparable to that of gnathostome T-cell and B-cell receptors by assembling variable leucine-rich repeat (LRR) modules. Thus far, three types of VLRs, designated VLRA, VLRB, and VLRC, have been identified in both lampreys (Kasamatsu et al 2010;Pancer et al 2004;Rogozin et al 2007) and hagfish (Li et al 2013;Pancer et al 2005).…”

Lamprey immunity is far from primitive

“…The extant agnathans are monophyletic and comprise two lineages: lampreys and hagfish (7). Two VLR paralogs have been identified in the hagfishes: a clear VLRB ortholog and a locus that is related to lamprey VLRA and VLRC, although the orthology is unresolved (6,8). It remains unknown whether a third VLR locus exists in the hagfish.…”